When people first sailed around the islands of Polynesia some 3500 years ago, rats often went along for the ride. Now, geneticists are using DNA from these rodents' descendants to trace human migration patterns on those remote scraps of land. Their data confirm suspicions that most early Polynesians shared a "homeland region" of islands, but that one or two groups lived in isolation.

So far, researchers have largely relied on linguistic and archeological evidence to map out human migrations in prehistoric Polynesia. Genetic analysis of modern humans is difficult, in part because the island populations were decimated by European diseases at the end of the 19th century. But the Polynesian rat, which came along on most of the colonization voyages, either as a stowaway or as a welcome passenger and food source, is alive and well. So Lisa Matisoo-Smith and her colleagues at the University of Auckland in New Zealand decided to look at genetic relationships among rats from the various islands. They focused on a 450-base pair section of mitochondrial DNA, because such DNA mutates at a steady rate regardless of the pressures of natural selection.

When they divided the rats into groups based on their genetic similarity, their results confirmed a few of the commonly held beliefs about Polynesian migration. First, rats from the Southern Cook and Society Islands were spread throughout many of the groups, strengthening the idea that this area served as a homeland region, from which people traveled to other islands. "This is clear evidence that people were moving around quite a lot," says Matisoo-Smith. One exception was the rats from Chatham Island, which clearly belong to a single related group, confirming that they were isolated after the initial settlement, says Matisoo-Smith.

The rat genetics approach is "highly innovative," says Patrick Kirch, an anthropologist at the University of California, Berkeley. "Rats have a higher mutation rate [than humans], so you get these different [families] developing in Polynesia," he says. "It's nice to have an independent confirmation of the work that's been coming out of archaeology and linguistics," he says.